The present invention relates generally to metal Schottky junction field effect transistors (MESFET) and more specifically to an isolated top gate MESFET and a method of making.
MESFETs are junction field effect transistors in which at least one gate is formed by use of a Schottky barrier diode rather than by an PN junction diode. The performance of such devices can be improved when they are built such that the two gates are isolated from one another. This is particularly valuable in integrated circuit applications where one of the gates only weakly modulates the channel and has significant junction leakage and/or capacitance associated therewith. The weak, parasitic loaded gate may be connected to a non-sensitive circuit node, often the source, while the other gate is used as a device control gate thereby reducing the parasitics on the control gate.
Another advantage afforded by an isolated gate structure is that several devices can be built in a common bottom gate isolated island rather than in separate isolated islands. This saves die area and improves match of matched pairs by allowing the members of the pair to be closer to one another.
Isolated gate devices can also be used as four terminal devices in which the second gate is the fourth terminal. Such devices permit novel circuit implementations not possible with a three terminal device. An example is use of the second gate to apply an automatic gain control signal to a field effect transistor being used as an amplifier device.
Several prior art methods which form isolated top gate devices are known. One uses an insulator or semi-insulating support region under the channel to eliminate the second junction thereby transforming the bottom gate into an MOS gate. Another method uses a closed geometry top gate in which the top gate encloses at least one of the source and drain contact regions. The enclosed region or regions is then connected by a second level of interconnect. These methods are difficult and expensive to produce, thus there is a need for improved methods.
For PN junction field effect transistors having thin channel regions and top gate ohmic contact regions, special processing must be produced to provide an appropriate top gate on the contact region isolated as illustrated in U.S. Pat. Nos. 4,456,918 and 4,495,694 to Beasom.
In MESFETs having a rectangular Schottky barrier diode top gate on the channel between two non-concentric source and drain regions, the designer must terminate the width of the top of the Schottky metal spaced from the edge of the channel in order to produce an isolated top gate. This separation is determined by the accuracy of forming the necessary apertures in the oxide such that it does not extend outside of the channel. The separation prevents the field effect transistor from being turned completely off and creates a parasitic field effect transistor having only a bottom gate which is in parallel with the main field effect transistor. This problem also occurs where the device is built in a dielectrically isolated island since the lateral dimension of the island varies because the manufacturing technique. Thus, although it creates an isolated top gate, it also produces a transistor which may not be applicable for all applications.
Thus, it is an object of the present invention to provide an isolated top gate MESFET which is capable of being turned completely off.
A further object of the present invention is to provide a method of forming an isolated top gate MESFET which can be turned completely off.
A still even further object of the present invention is to provide a method of forming an isolated top gate MESFET in a dielectrically isolated region island.
These and other objects of the invention are attained by using a Schottky top gate which extends across the channel region between the source and drain regions and beyond two opposed sides of the dielectric isolation onto the substrate in which the device is built. The portion of the top gate which extends across the channel is disconnected from the portion which extends across the substrate beyond the dielectric isolation. This may result from the removal of the gate material at the dielectric isolation or by the portion of the gate material which is on the dielectric isolation being vertically displaced and disconnected or discontinuous from the portion of the gate material which extends across the channel and that portion which extends across the substrate. The bottom gate also extends beyond the dielectric isolation below the surface of the island and intersects the bottom of the source and drain regions. The length of the Schottky barrier top gate and the bottom gate diffusion are sufficiently large so as to extend beyond the dielectric isolation for the maximum anticipated island size which results from the dielectric isolation process. Thus, the top and bottom gates completely define the channel and prevent any leakage current beyond the gates.
One method of forming a disconnection of the gate material between the portion extending across the channel and the portion extending across the substrate includes removing opposed portions of the dielectric isolation below the surface of the substrate. Applying a gate material on the top surface of the island extending across the channel and onto the substrate of a thickness small enough compared to the depth of the removed dielectric isolation to be disconnected at the dielectric isolation. Preferably, the width of the removed dielectric isolation is greater than the width of the top gate formed.
Another method of forming the disconnected top gate portion includes applying a top gate material whose silicide will form a Schottky barrier top gate with the channel, over the channel region, the dielectric isolation and the substrate and heating to alloy the gate material with the island and the substrate. The unalloyed portion of the gate material on the dielectric isolation is selectively removed to form a silicide Schottky barrier top gate with and extending totally across the channel and being discontinuous at the dielectric isolation from the silicide over the substrate.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.